KKR is correct that span
bridges (as opposed to hexside
bridges) may not be destroyed. However, scenario designers can 'fake' blowable span
bridges by adding a river hexside across the end of a span bridge and then placing a hexside bridge across that river. In this way a player can blow the hexside bridge and the span bridge becomes impassable.
So, to make sure that a span bridge is not blowable check out whether it has a river hexside at either and and a hexside bridge over it.
umbro
P.S.
NOT BLOWABLE
BLOWABLE